This invention is directed to a crossed-field switch device, and in particular a method and apparatus for on-switching the crossed-field switch device when high voltage is applied thereto.
From the original Penning work on glow mode discharge in an interelectrode space where the magnetic field is at an angle to the electric field evolved the structure of U.S. Pat. No. 2,182,736. A considerable amount of development work has been done at the Research Laboratories of Hughes Aircraft Company to develop the crossed-field glow mode discharge into a switch device which is capable of off-switching large current against high voltage. The off-switching speed is so rapid that off-switching can occur between natural current zeros of the usual 60 cycle power line. When the off-switching device is very important for direct current off-switching, it is also applicable to rapid off-switching of power line alternating current between natural current zeros. General background along these lines is illustrated in G. A. G. Hofmann U.S. Pat. No. 3,604,977 as well as in H. E. Gallagher and W. Knauer U.S. Pat. No. 3,963,960.
In order to maintain a glow discharge in an interelectrode space, the path of an electron as it moves from one electrode to another through the gas in the interelectrode region must be sufficiently long that cascading ionization occurs. In other words, statistically each electron must have enough collisions to produce more than one ionizing collision. The maintenance of gas pressure and the lengthening of the electron path between the electrodes by the application of the crossed magnetic field is discussed in G. A. G. Hoffmann and R. C. Knechtli U.S. Pat. No. 3,558,960; M. A. Lutz and R. C. Knechtli U.S. Pat. No. 3,638,061; R. E. Lund and G. A. G. Hofmann U.S. Pat. No. 3,641,384; and G. A. G. Hofmann U.S. Pat. No. 3,769,537. Each of these patents shows the Paschen curve of voltage vs. the product pd where p is pressure and d is the interelectrode space. These curves are for a particular gas and zero magnetic field. The curves define regions between conductive and non-conductive conditions. They show that for a particular value of the product pd, the voltage at which breakdown into the glow mode occurs is at a minimum.
M. A. Lutz and G. A. G. Hofmann U.S. Pat. No. 3,678,289 discusses off-switching and discusses the characteristics of the glow mode discharge which permit off-switching. The patent shows in FIG. 3 a curve of the applied voltage across the interelectrode space vs. the magnetic field in the interelectrode space and shows the relationships of these parameters in which glow mode discharge does and does not occur, for fixed values of the product pd and for a particular gas. It is this V vs. B curve which shows the difficulty of on-switching when high-voltage is applied to the interelectrode space.
G. A. G. Hofmann U.S. Pat. No. 3,714,510 and M. A. Lutz and R. Holly U.S. Pat. No. 3,890,520 are both directed to on-switching of a crossed-field switch device by ionizing the gas in the interelectrode space. the application of ionization does not initiate a glow mode discharge and glow mode conduction when the initial conditions before the on-switching comprise a high interelectrode voltage and normal magnetic field. This is because the high interelectrode voltage captures electrons and draws them to the anode before the path length is sufficiently long to cause cascading ionization. The method of on-switching the crossed-field switch device of G. A. G. Hofmann U.S. Pat. No. 3,714,510 comprises the initiation of an interelectrode arc discharge to reduce the interelectrode potential, and after extinguishment of the interelectrode arc, the interelectrode potential is sufficiently low to initiate and permit conduction in the glow mode. The on-switching method of M. A. Lutz and R. Holly U.S. Pat. No. 3,890,520, while a high voltage is applied thereto, comprises the application of a sufficiently high over-all magnetic field to move the operating point to the right on the voltage vs. magnetic field curve to reach the conductive region even while the interelectrode voltage remains high.
This background illustrates the need for a method and apparatus for on-switching a crossed-field switch device during the application of high voltage to the electrodes, without arcing and without the need for a magnetic field source capable of very strong over-all magnetic fields.